The present invention relates to an active matrix liquid crystal display device driven by thin film transistors (TFTs), and an interconnection formation method for such a liquid crystal display device.
In recent years, the market for TFT-driven liquid crystal display devices (TFT-LCDs) has expanded, compared with conventional CRTs, as they have recognized as image display devices capable of realizing further reduction in size and weight and improvement in definition. Such a TFT-LCD mainly includes: a glass substrate on which gate lines, data lines, TFTs formed at the respective intersections between the gate lines and the data lines, pixel electrodes connected to the TFTs, a gate insulating film, and a protection film are formed; a counter substrate; and a liquid crystal layer sandwiched by the glass substrate and the counter substrate.
With the recent progress in the achievement of a larger screen size and higher definition of TFT-LCDs, the requirements for lower resistance of interconnections and lower production yield have become stricter. For lowering the resistance of interconnections, aluminum or an aluminum alloy has been conventionally used. However, when a single layer of aluminum or an aluminum alloy is used for interconnections, hillocks tend to be generated on the surface of the interconnections, resulting in that an insulating film formed over the interconnections fails to cover them sufficiently (coverage failure). Moreover, aluminum or an aluminum alloy exhibits high contact resistance against an indium-containing oxide used as the material of pixel electrodes, such as indium tin oxide (ITO) and indium zinc oxide (IZO). It is therefore impractical to attempt direct electrical connection therebetween. In order to overcome these problems, in an inverted-stagger type TFT-LCD, gate lines are formed of a cladding structure in which a metal having a high melting point covers a wiring pattern of aluminum or an aluminum alloy. In this cladding structure, the second conductive layer (metal) covering the wiring pattern of aluminum or an aluminum alloy provides the contact characteristics with the pixel electrode material, while the aluminum or an aluminum alloy provides the conductivity as the interconnections. Such a structure is disclosed in, for example, Japanese Patent Laid-Open Nos. 341299/1993, 64109/1995, 26602/1997, 127555/1997, and 213809/1998.
The formation of the above cladding structure requires a complicate process including two photolithographic steps, one for the aluminum or aluminum alloy and the other for the second conductive layer. In order to make the process simpler, the layer of aluminum or an aluminum alloy and the second conductive layer are sequentially formed and etched at one time in one photolithographic step to form a wiring pattern. For this procedure, molybdenum or a molybdenum alloy is used for the second conductive layer since molybdenum is a metal having a high melting point allowing for one-time etching with aluminum or an aluminum alloy. Japanese Patent Laid-Open No. 20930/1992, particular, discloses an interconnection structure using a molybdenum alloy containing 0.5 to 10 wt % of chromium as the second conductive layer. In this disclosure, the layered structure is wet-etched at one time with a mixed solution of phosphoric acid, nitric acid, and acetic acid. The section of the resultant interconnection has a taper angle of 50xc2x0. Such a molybdenum-chromium alloy has a resistance against dry etching with a fluoric gas such as SF6. This provides an advantage as follows. When a contact hole or the like is formed through an overlying SiN insulating film by dry etching with SF6 gas, the second conductive layer made of the molybdenum-chromium alloy will not be etched away at the bottom inside the contact hole, ensuring connection with the pixel electrode and thus providing good contact characteristics with the pixel electrode.
There is a report on examinations on the sectional shapes of an interconnection of a molybdenum/aluminum layered structure when etched by a dip method and a shower method (Digest of Technical Papers of 1994 International Workshop on Active-Matrix Liquid-Crystal Displays, Nov. 30-Dec. 1, 1994, Kogakuin University, Shinjuku, Tokyo, Japan, p.188). More specifically, the molybdenum/aluminum layered structure was subjected to one-time etching with a mixed solution of phosphoric acid and nitric acid by the dip method and the shower method, and the sectional shapes of the resultant interconnection obtained by the two etching methods were examined. According to this report, in the dip method, the section of the interconnection was tapered, while in the shower method, the molybdenum layer protrudes like a brim relative to the aluminum layer.
Japanese Laid-Open Patent Publication No. 331066 of 1997 discloses the use of titanium, molybdenum, tantalum, tungsten, zirconium, or a composite material thereof as the second conductive layer. According to this disclosure, a light-shading film and interconnections are simultaneously formed in the same process, with the second conductive layer playing a role of minimizing the reflectance of the light-shading film. No specific combination of elements for the composite material when selected as the second conductive material is mentioned in this disclosure.
Japanese Patent Laid-Open No. 258633/1999 discloses a fabrication method of an array substrate of a display device in which the second conductive layer is formed of a metal selected from chromium, molybdenum, tungsten, titanium, zirconium, hafnium, vanadium, niobium, and tantalum or an alloy thereof. According to this disclosure, the formation of such a second conductive layer prevents generation of a hillock on an aluminum alloy film and also prevents corrosion of the aluminum alloy during dry etching of pixel electrodes. No specific combination of elements for an alloy or the composition of such an alloy when selected as the second conductive layer is mentioned in this disclosure.
For enhancing the productivity of the array substrate of a liquid crystal display device, the size of the mother glass substrate has become larger. For example, according to Flat Panel Display 2000, Nikkei BP, P.56 (1999), the production lines for substrates of 590xc3x97670 mm2, 600xc3x97720 mm2, and 650xc3x97830 mm2 were in actual operation in 1998, and in the year of 2000, production lines for 680xc3x97880 mm2 and 730xc3x97920 mm2 are expected to be in operation. To correspond to such size increase of the mother glass substrates, the size of the fabrication equipment has become larger. In the case of a wet etching apparatus, as the size of the apparatus increases, sufficient stirring of an etchant becomes difficult in the dip method, and thus it is almost impossible to realize uniform etching over a large-area substrate. In order to obtain highly uniform wet etching over a large-area substrate, therefore, the shower method must be employed. However, the shower method has the problem described above. That is, in the shower method, when a layered structure of a molybdenum layer and an aluminum layer is etched at one time with a mixed solution of phosphoric acid and nitric acid, the molybdenum layer protrudes like a brim relative to the aluminum layer. This sectional shape of the interconnection was also confirmed by the present inventors in an experiment where a layered structure composed of a molybdenum alloy layer and an aluminum alloy layer was etched at one time with a mixed solution of phosphoric acid, nitric acid, and acetic acid by the shower method. If an insulating film is formed over the resultant interconnection having such a sectional shape, the insulating film may generate coverage failure, resulting in lowering the production yield.
In view of the above, the first challenge of the present invention is to obtain a tapered sectional shape of the interconnection having a layered structure of a molybdenum alloy and an aluminum alloy by the shower wet etching method thereby to ensure good coverage of the overlying insulating film. The second challenge of the present invention, which should be solved together with the first challenge, is to provide the second conductive layer with a resistance against dry etching with a fluoric gas such as SF6 so that when a contact hole or the like is formed through an overlying SiN insulating film, the second conductive layer will not be etched away at the bottom inside the contact hole, securing connection with the pixel electrode. In the case of one-time etching of the layered structure of a molybdenum alloy as the upper layer and an aluminum alloy as the lower layer, the side faces of the lower aluminum alloy layer are exposed. Hillocks may be generated on such faces. The third challenge of the present invention is to minimize generation of such hillocks of the aluminum alloy. High production efficiency and securement of a process margin as large as possible are also attempted to attain in the present invention.
The first means for solving the above first and second challenges simultaneously is to use, as the second conductive layer, such a material that has a resistance against dry etching with a fluoric gas such as SF6 equal to or higher than that of the molybdenum-chromium alloy and also has a wet etching rate sufficiently higher than that of the molybdenum-chromium alloy for an etchant used in the one-time etching with the aluminum alloy.
In order to find such a material, the present inventors have prepared various molybdenum alloys containing any of chromium, titanium, tantalum, zirconium, and hafnium at a variety of concentrations, and measured the dry etching rates for SF6 gas and the wet etching rates for a mixed solution of phosphoric acid, nitric acid, and acetic acid for the various molybdenum alloys. The results are as shown in FIG. 20, in which the X-axis represents the wet etching rate and the Y-axis represents the dry etching rate.
All the molybdenum alloys exhibited curves indicating that both the wet etching rate and the dry etching rate decrease as the concentration of the added element in the alloy increases. The detected lower limit value of the dry etching rate was 0.02 nm/s. As for the molybdenum-tantalum alloy, the dry etching rate little decreases while the wet etching rate greatly decreases in comparison with the molybdenum-chromium alloy. Therefore, the molybdenum-tantalum alloy is not suitable for the purpose of the present invention. This is also applicable to a molybdenum-tungsten alloy and a molybdenum-niobium alloy. On the contrary, as for the molybdenum-zirconium alloy and the molybdenum-hafnium alloy, the dry etching rate greatly decreases while the wet etching rate does not decrease so largely in comparison with the molybdenum-chromium alloy. These alloys are therefore suitable for the purpose of the present invention. Although not shown in the figure, vanadium-added molybdenum alloy has also an effect of greatly reducing the dry etching rate.
In the case of using the second conductive layer for gate lines, the required dry etching resistance of the second conductive layer is such that the etching selectivity ratio of an overlying SiN layer to the second conductive layer is 7 or more. Since the dry etching rate of SiN for SF6 gas is 19.4 nm/s, this requirement on the dry etching resistance will be satisfied if the dry etching rate of the second conductive layer is 2.78 nm/s or less. The amount of the added element required to obtain this dry etching rate: is 2.6 wt % or more for zirconium and 4.9 wt % or more for hafnium. In the case of using the second conductive layer for drain lines, the required dry etching resistance of the second conductive layer is such that the etching selectivity ratio of the overlying SiN layer to the second conductive layer is 14 or more. Since the dry etching rate of SiN for SF6 gas is 19.4 nm/s, this requirement on the dry etching resistance will be satisfied if the dry etching rate of the second conductive layer is 1.39 nm/s or less. The amount of the added element required to obtain this dry etching rate is 4.0 wt % or more for zirconium and 7.3 wt % or more for hafnium.
In order to obtain a tapered sectional shape of the interconnection with the layered structure of the aluminum alloy layer and the second conductive layer by one-time wet etching of both layers, the second conductive layer is required to have a wet etching rate equal to or higher than that of the aluminum alloy. The amount of the added element necessary to satisfy this requirement is 23 wt % or less for zirconium and 36 wt % for hafnium. In order to secure a sufficient margin for tapering control in the shower etching method, the wet etching rate 2.4 times as high as that of the aluminum alloy is required. The amount of the added element necessary to satisfy this requirement is 14 wt % or less for zirconium and 22 wt % or less for hafnium. The aluminum alloy used is assumed to be Al-9.8 wt % Nd of which wet etching rate is 5.1 nm/s. The effect obtained by the molybdenum-zirconium alloy and the molybdenum-hafnium alloy described above is also obtained by a molybdenum-zirconium-hafnium ternary alloy. In some cases, a properly low wet etching rate may rather be better in the tapering control depending on the wet etching conditions and the constitution of the layered film. In such a case, an appropriate amount of chromium may be added to the molybdenum-zirconium alloy and the molybdenum-hafnium alloy for controlling the wet etching rate.
In the case of the molybdenum-titanium alloy, the dry etching rate decreases slightly largely compared with the decrease of the wet etching rate, in comparison with the molybdenum-chromium alloy. The molybdenum-titanium alloy is therefore applicable to the purpose of the present invention although the effect is not so great as that provided by the molybdenum-zirconium alloy and the molybdenum-hafnium alloy. The required amount of titanium is 2.3 wt % or more for gate lines and 3.4 wt % or more for drain lines. The amount of titanium required for obtaining a wet etching rate equal to or higher than that of the aluminum alloy is 6.7 % or less. The amount of titanium required for securing a sufficient margin of tapering control is 4.0 wt % or less. The molybdenum-titanium alloy has an additional effect of securing good electrical contact with the aluminum alloy even when the molybdenum-titanium alloy is formed on the aluminum alloy that is covered with an atmospherically generated oxide film. This is due to the ability of titanium of taking the oxygen out of the aluminum oxide. Therefore, when the layered structure is composed of an aluminum alloy and a molybdenum-titanium alloy, it is not necessarily required to form these layers sequentially in the vacuum state. This serves to ease restraint in production. It should be understood that the molybdenum-chromium alloy fails to satisfy securing the dry etching resistance when used for drain lines and securing a sufficient margin of tapering control simultaneously by the shower method.
The second means for solving the first and second challenges simultaneously is to form interconnections by adjusting the composition of an etchant for one-time etching of the aluminum alloy and the second conductive layer. Specifically, the mixed etchant of phosphoric acid (H3PO4), nitric acid (HNO3), acetic acid (CH3COOH), and water (H2O) is adjusted to include the nitric acid in the range between 7 mol % and 12 mol % inclusive and at least one of ammonium fluoride (NH4F) and Hydrogen fluoride (HF) in a trace amount of 0.01 to 0.1 mol %. The presence of nitric acid at the above concentration causes the ends of a resist pattern to roll up. This serves to increase the etching rate of the second conductive layer on the portions in contact with the resist (side etching rate), and thus allows for etching into a tapered sectional shape by the shower method. The addition of a trace amount of ammonium fluoride or hydrogen fluoride minimizes generation of etching residues on the surface of the aluminum alloy. In addition, in-plane uniformity of the tapered shape is improved by shaking shower nozzles of an etching apparatus . By using the etchant with the above composition for shower etching, tapering is possible even when the wet etching rate of the molybdenum alloy is 3.8 nm/s, a little lower than that of the aluminum alloy. To obtain this wet etching rate, the upper limit of the amount of the added element to molybdenum is 3.0 wt % for chromium, 26 wt % for zirconium, 41 wt % for hafnium, and 7.6 wt % for titanium.
The third challenge can be solved by using an aluminum alloy containing neodymium in an amount of 0.2 at % or more, preferably 2 at % or more. In the case where the layered structure is used for the gate lines, the second conductive layer is not required in the pixel portions of the gate lines, but only required in the terminal portions thereof for securing connection with the pixel electrodes. Therefore, in the pixel portions, the aluminum alloy may be anodized. This minimizes the coverage failure of the insulating film due to hillocks generated on the aluminum alloy and the like. In the case where the layered structure is used for the drain lines, a third conductive layer may be formed under the aluminum alloy layer to secure contact with the underlying semiconductor layer. In this case, amorphous indium tin oxide (a-ITO) or indium zinc oxide (IZO) that allows for use of a weak-acid etchant is preferably used as the material of the pixel electrodes so that the aluminum alloy is prevented from being damaged during etching of the pixel electrodes. If the aluminum alloy and the second conductive layer are shared between the gate lines and the drain lines, the number of sputtering targets required for fabrication of the array substrate of the liquid crystal display device is reduced, and the degree of freedom in the operation of the sputtering apparatus improves, providing an advantage in the aspect of production. If the third conductive layer for the drain lines is also shared, the effect will become greater. Alternatively, the drain lines may be composed of a single molybdenum alloy layer. This constitution having no aluminum alloy eliminates the necessity of considering possible damage on the interconnections during etching of the pixel electrodes, allowing for use of polycrystalline indium tin oxide (poly-ITO) having high reliability as the pixel electrodes. As the molybdenum alloy used as a single layer, a molybdenum-chromium alloy is suitable since it exhibits both high dry etching resistance and low resistivity. The required dry etching resistance is such that the etching selectivity ratio of SiN to the Mo alloy is 3.5 or more. The amount of chromium necessary to satisfy this requirement is 0.35 wt % or more.
In the case where the layered film of a Mo-8 wt % Zr alloy and an Alxe2x80x94Nd alloy according to the present invention is used for gate lines (gate lines), the upper Mo-8 wt % Zr alloy layer may be removed electrochemically and the Alxe2x80x94Nd alloy may be anodized to selectively form an aluminum oxide layer as the upper layer of the gate lines. This greatly improves reliability of the insulation and resistance of the gate insulating film.